I'm not saying that people should just go ahead and inhale diacetyl freely, but to add some perspective... It's my understanding that the lung damage came from people in a factory inhaling large amounts of the stuff all day long, day after day. NF said that there are trace amounts of it (naturally occurring) in some flavors.
That's what I understand also. If you go to the health section on ECF, there are a few threads on it. No, I'm sure no one wants anything dangerous in our liquids but at the same time, I would think we would have to vape a ton for it to have an effect on our lungs.